16ft Heat Exhange too short??

Is a 6inch diameter 16ft long heat exchange horizontal exhaust with 3 90° too short??

It's probably on the short side for heat exchange, but with the elbows is probably about maxed out for friction loss. If you have a great chimney with natural strong draft, you could probably safely make it a bit longer.

I would advise putting a half-barrel bell in the place where you double back with elbows. This would reduce friction losses and considerably increase heat exchange effectiveness at the same time.

Another trick to increase exchange surface, without lengthening the pipes, is to use  bigger diameter pipes. Also slows the gases down, so you get more exchange time too.

And next step, is to use a bell.

Yeah im just limited on space, my chimney will be class A near the bell and about 25ft indoors and a few more feet past the peak. Ive just never researched with running half bells as the heat exchange as others have recomended so it makes me a bit weary to try it as I already have all my 6 inch piping at hand

It's all up to you.

One thing to know, is heat exchange takes time and surface. The more time the gases stay in contact with colder materials, the more time they have to exchange heat.

The more surface they have to exchange heat, the better, until you reach dew point, where it's catastrophic.

Im going to try to bring it up to 20ft the best I can. I feel like that would bd about perfect. If iI cant il do more research on using bells. When the book talks about using perlite is that the same stuff that you use in your garden??

Garden perlite is the same.

Basically, yes. There are different types of perlite, mostly due to granule size as far as I know, but they all work.

A half barrel bell is about as simple as you can get construction-wise. Cut a (steel) barrel in half lengthwise, lay it cut side down, arrange it so the duct entrance and exit are near the bottom and cut holes to fit. You want the entrance not to point straight at the exit so you don't short-circuit the bell volume.